The increasing prevalence of obesity in U.S. children and adolescents is a major health threat to our society, especially among minority and low social economic status (SES) populations. During adolescence physical activity (PA) decreases and is likely an important contributor to the increasing trend in childhood obesity rates. Little evidence suggests that school-based curriculum interventions lead to increases in overall PA. Thus, this proposal will evaluate the efficacy of an innovative student-centered after-school program for promoting increases PA among underserved adolescents (e.g., minorities, low SES). The student-centered intervention is consistent with Self-Determination (Motivation) Theory and Social Cognitive Theory in that it emphasizes increasing intrinsic motivation and behavioral skills for PA. Adolescents in the student-centered program take ownership in developing the program, select physical activities that generate fun and interest, and generate their own coping strategies for making effective PA changes during a videotaped session. Preliminary data from our group demonstrates the feasibility of the student-centered PA program for increasing moderate-to-vigorous PA in underserved adolescents in South Carolina. The proposed project will use a school-based nested cohort design to evaluate efficacy of a 6-month student-centered program versus typical after-school program on ncreasing PA in underserved adolescents. Twenty-four middle schools (70 6th graders per school; N=1,680), located in South Carolina will be randomly assigned to one of two after-school programs. The study employs a nested cohort design, with schools, rather than individuals assigned to condition and will be analyzed using repeated measures analysis of covariance techniques as outlined by Murray. We will also examine psychosocial variables (PA self-efficacy, self-concept, motivation, social support, and enjoyment) as potential mediators of the intervention on changes in PA using regression and structural equation modeling techniques as outlined by Baron and Kenny. This study will address an important public health problem that will have implications for decreasing obesity in underserved adolescents.